1. Field of the Invention
The present invention relates to a magnetic disk drive, for use as a memory device in a laptop or book-type portable electronic apparatus, and an assembling method therefor.
2. Description of the Related Art
A magnetic disk drive with large memory capacity and high-speed access performance is conventionally known as a memory device which is used in a laptop or book-type portable electronic apparatus. For example, a hard disk drive (hereinafter referred to as HDD) comprises a magnetic disk, a motor for rotating the disk, a magnetic head for data recording and retrieval for the disk, a carriage supporting the head, etc. These components are contained in a housing.
Each magnetic disk has a hole in the center. The motor is provided with a cylindrical rotor which rotates about a motor shaft. The rotor is fitted in the center hole of the magnetic disk. The disk is fixedly supported on the rotor by means of a fixing member.
Recently, there has been an increasing demand for high-performance, large-capacity HDDs. This demand can be met by increasing the rotational speed of the magnetic disk or the motor speed and the number of the disks mounted on the rotor of the motor.
When the magnetic disk is mounted on the rotor of the motor and rotated together with the rotor, in one such HDD, the rotation of the disk is unbalanced if the disk is eccentric to the center of rotation of the rotor, and this unbalance causes the rotor and the disk to vibrate. If the motor speed and the number of the magnetic disks mounted on the rotor are increased as aforesaid, influences of the unbalanced rotation of the disks are enhanced, so that the vibrations of the rotor and the disks attributable to the rotation further increase.
In some cases, the increase of the vibrations of the rotor and the disk may exert a bad influence upon the performance of the electronic apparatus which is furnished with the HDD, as well as upon the positioning accuracy of the magnetic head with respect to the disk. In the case of a compact electronic apparatus, such as a book-type personal computer, the HDD may vibrate to cause vibrations of a keyboard, thereby giving an operator an unpleasant feeling.
In the case of a 2.5-inch magnetic disk, for example, a gap of about 5 to 90 .mu.m is generally formed between the inner circumferential edge of the disk, which defines the center hole, and the outer circumferential surface of the rotor. Owing to this gap, the magnetic disk and the rotor become eccentric to each other, thus causing the rotation of the disk to be unbalanced.
The unbalance force (centrifugal force) for each disk is represented by F=m.multidot.e.multidot..omega..sup.2, where m, e, and .omega. are the mass, eccentricity, and rotational speed of the magnetic disk, respectively. If aluminum disks with a thickness of 0.889 mm are rotated at the speed of 3,600 rpm with the gap of 90 .mu.m between the edge of each disk center hole and the outer circumferential surface of the rotor of the motor, the unbalance force for each disk, based on the aforesaid equation, is EQU F={1/4(65.sup.2 -20.sup.2) .times..pi..times.0.889.times.2.7.times.10.sup.-6 }.times.(90.times.10.sup.-6 /2).times.(2.pi..times.60).sup.2 =0.046N.
If the weight M of the HDD is 200 g, vibrations of the HDD caused by the unbalance force, based on F=M.alpha. (.alpha. is acceleration), is .alpha.=F/M=0.046/200.times.10.sup.-3 =0.23 m/S.sup.2.
As the rotational speed and the number of the mounted magnetic disks increase, the unbalance force and the vibrations increase correspondingly.
Described in Jpn. Pat. Appln. KOKAI Publication No. 2-79278 (Fujitsu, Ltd.), for example, is a disk drive in which the outer circumferential surface of the rotor has a special shape such that a rotor and disks can be fixed concentrically. More specifically, the rotor has an outer circumferential surface with an outer diameter smaller than the inner diameter of the disks and a pair of projections protruding from the outer circumferential surface. The extending end face of each projection is in the form of a circular arc whose diameter is equal to the inner diameter of the disks. Each disk is aligned with the rotor by causing its inner circumferential edge to abut against the projections. A counterweight for balancing with the projections is provided on that region of the rotor on the opposite side thereof to the rotor.
If the rotor or the disks in this conventional arrangement are subject to a dimensional error as a manufacturing error, however, it is difficult to align the disks perfectly with the rotor. In an apparatus which uses a large number of disks or in which the disks are rotated at high speed, therefore, the rotor and the disks are unbalanced in rotation, and generate vibrations.
According to a magnetic disk drive disclosed in U.S. Pat. No. 6,483,505, moreover, a plurality of disks are fixed to a rotor in a manner such that they are alternately shifted in the diametrical direction of the rotor so that their respective inner circumferential edges abut against the outer circumferential surface of the rotor in positions situated at intervals of 180.degree. in the circumferential direction, whereby the disks are balanced.
In the case where an odd number of disks are used in this arrangement, however, unbalance for one disk is sure to remain and appear as vibrations when the disks are rotated.
Thus, according to the conventional arrangement, no effective measure is taken to meet the unbalance force produced by the rotation of the magnetic disks.